Glaucoma is a disease characterized by high pressure inside the eye, leading to the loss of retinal nerve fibers with a corresponding loss of vision. Glaucoma, therefore, is a disease affecting the optic nerve, the nerve bundle which carries visual information to the brain.
The eyeball is basically a rigid sphere filled with fluid. Positive pressure inside the eye is needed to keep the retina attached to the back of the eye. Pressure is maintained by fluid production from a bilayer of cells on top of the ciliary body, which is located adjacent to the iris root in the eye. This clear fluid, called aqueous humor, carries nutrients to the lens and cornea of the eye, both of which have no blood supply. The shape of the front part of the eye is maintained by aqueous humor production. The ciliary body is located behind the colored part of the eye (iris). Fibrous strands called zonule fibers attached to the ciliary body support the lens. Tension from the rigid structure of the eye, transferred to these zonules, deforms the lens and focuses the image onto the retina. Radial muscles behind the ciliary body contract and release tension on the zonules allowing the lens to round up and focus near images onto the retina. Aqueous humor flows into the front of the eye through the pupil and drains out of the eye through the trabecular meshwork. The trabecular meshwork is a spongy mass of tiny channels located in the drainage angle, between the clear covering of the eye (cornea) and the colored part (iris) at the location where the iris meets the white outer covering of the eye (sclera). The fluid drains from the meshwork into a small canal, called Schlemm's canal, which is connected to the bloodstream at the venous return from the eye. Aqueous humor is produced by the ciliary body and removed from the eye at a constant rate (about 1.0 tsp or 5.0 ml per day) to maintain a constant pressure in the front (anterior) chamber of the eye. While pressure in the eye varies throughout the day, the average pressure within the eye is 10 mm Hg to 15 mm Hg. If the resistance to fluid flow increases or the amount of fluid generated exceeds the capacity of the meshwork to vent it, pressure inside the eye increases, similar to over-inflating a tire. The higher the pressure inside the eye, the greater the chance of damage to the optic nerve. Glaucoma is the leading cause of blindness in third world countries, and the leading cause of preventable blindness in industrial countries. It affects approximately 2% of the entire population; blacks and native Americans are disproportionately represented, with elevated occurrence of the disease. Early signs of the disease are often observed as enlargement and cupping of the physiological blind spot which is the point where the optic nerve leaves the eyeball and projects to the brain. Blind spots in the superior and inferior visual fields (called arcuate scotomas) correspond to the loss of nerve cells. In later stages, more visual field losses eventually end in total blindness. If the drainage angle becomes blocked, fluid pressure transferred throughout the eye increases, damaging the optic nerve—the part of the eye responsible for transforming images into impulses the brain can translate. This damage results in partial or complete blindness.
In acute angle-closure glaucoma, eye pressure builds up rapidly. This type of glaucoma commonly occurs in individuals who have narrow anterior chamber angles. In these cases, aqueous fluid behind the iris cannot pass through the pupil and pushes the iris forward, preventing aqueous drainage through the angle. It is as though a sheet of paper floating near a drain suddenly drops over the opening and blocks the flow out of the sink. In cases of acute angle closure glaucoma, one may experience blurred vision, halos around lights, pain, nausea, and vomiting. If pressure within the eye is not immediately relieved, blindness may result in a matter of days. Migration of pigmented epithelial cells in the eye, either congenital or following blunt trauma, can occlude angle structures quickly elevating pressure in the eye. Primary open-angle glaucoma has a longer time course and many components that exacerbate the condition. The end effect is the same. Secondary glaucoma can occur from inflammation, degeneration, trauma, or tumor growth within the eye. Certain medications are also reported to cause secondary glaucoma. In summary, the disease glaucoma is a plethora of conditions that elevate intraocular pressure. Left uncontrolled, high intraocular pressure leads to blindness.
In the detection of glaucoma, measuring the pressure in the eye by itself does not give a diagnosis of glaucoma. Of more importance is the direct observation of damage to the optic nerve itself and sometimes the nerve fiber layer of the retina. Quantifying a loss of vision in part of the visual field consistent with observed nerve fiber loss is the true definitive diagnosis for glaucoma. Loss of nerve fibers caused by glaucoma cannot be reversed. Therefore, the goal in the management of glaucoma is to reduce the intraocular pressure to the point whereby the remaining healthy nerve fibers are able to maintain function.
Surgical treatment of glaucoma using a shunt implant to vent aqueous humor from the anterior chamber to a subconjunctival bleb is warranted in some cases. To date, all the currently available shunt implants consist of a tube or conduit attached to the surface of a large plate. The conduit carries fluid from the anterior chamber to the surface of the plate. A cellular capsule forms around that plate and once inflated with aqueous humor from the anterior chamber forms a large blister or bleb around the plate on the outer surface of the eye. The geometry of the blister's surface takes on the shape of the plate. The cellular capsule forming the bleb wall filters fluid to the subconjunctival space and provides resistance to fluid flow so pressure in the eye will not be too low.
Our research shows that a large volume blister causes most of the postsurgical complications. A hemispherical blister or any large cavity is an ideal geometry for maximizing volume while minimizing surface area. Since the bleb wall filters fluid, the surface area and hydraulic conductivity of that filter are the important parameters in its function. In filtration across a membrane, the pressure across the filtration capsule and the surface area of the filtration capsule determine how much fluid will filter across the membrane. Since increasing surface area is the goal, a spherical design is an adverse geometry for this goal.
Another problem with spherical blister geometry is that surface tension is proportional to the radius of a blister (Laplace's law). The larger the force applied to this cellular capsule, the more the membrane will stretch out until the cohesive force of the capsule can no longer match the force provided by pressure, in which case it will rupture causing the pressure to be too low. The body's reaction to applied force is to reinforce the capsule with collagen. All large plate devices experience this condition and require a maturation period during which the capsule thickens but a thick capsule makes it less effective at filtering or venting fluid and sometimes fails early due to the build up of fibrous tissue in the capsule.